This invention relates to hybrid circuits for providing an interface between a pair of unbalanced unidirectional transmission paths and a balanced two-wire, bidirectional transmission path. More particularly this invention relates to hybrid circuits of such type which are direct coupled, i.e. transformerless.
Hybrid circuits are known for providing signal conversion between a balanced two-wire, bidirectional transmission path and a pair of unbalanced unidirectional transmission paths, the latter comprising a first unidirectional transmission path for carrying incoming electrical signals and a second unidirectional transmission path for carrying outgoing electrical signals. Such hybrid circuits are typically employed in the telephone art to provide a signal interface between a telephone carrier channel having the aforementioned pair of unidirectional transmission paths and the balanced bidirectional transmission paths at either end of the carrier channel to which are connected the subscriber loop and the telephone instrument at one end and the central office equipment at the other end.
In the past, hybrid circuits have been conventionally implemented by means of transformers having a pair of loop terminals connected to the bidirectional signal path and terminals connected to the separate incoming and outgoing unidirectional signal paths.
Due to limitations inherent in transformer type hybrids, recent efforts have been directed to the design of transformerless hybrid circuits having substantially similar functional capability as the transformer type hybrids. State of the art hybrids of this type, of which the disclosure of U.S. Pat. No. 3,849,609 is representative, employ semiconductor current controlled current sources hereinafter designated "current mirrors" to convert incoming signals from one unidirectional transmission path to balanced signals on the bidirectional transmission path and to convert balanced signals originating in the bidirectional transmission path to outgoing signals on the other unidirectional transmission path. Circuits of this type possess the decided advantage of being implementable as integrated circuits. This results in lower manufacturing costs than for the transformer type hybrids, together with good reliability and small physical size.
In U.S. Pat. No. 4,004,109 there is described a transformerless hybrid for providing signal conversion between a balanced bidirectional transmission path and two unidirectional transmission paths, for suppressing longitudinal (common mode) signals originating in the bidirectional transmission path and for providing signal balance to suppress transmission of reflected echoes from the incoming unidirectional transmission path to the outgoing unidirectional transmission path.
Of the several embodiments disclosed in said patent, each includes a plurality of current mirror circuits, some of which are of complimentary conductivity type to the remaining current mirror circuits. Each current mirror circuit has a common terminal, one input terminal, at least one output terminal, and means for supplying currents to the output terminals which are proportional to the current at the input terminal.